1. Field of the Invention
This invention relates to a packaging material suitable for photographic photosensitive materials.
2. Description of the Prior Art
Various types of packaging materials for photosensitive materials have been put to practical use, and various properties are required according to their uses.
As a packaging material for photosensitive materials, it is necessary to have various properties such as packaging material slitability, light-shielding, gas barrier, moistureproofness, antistatic property, rigidity, physical strength such as breaking strength, tear strength, impact puncture strength, Gelbo test strength and wear resistance, heat sealing properties such as heat seal strength, side welding (cut-sealability), hot tack properties (hot-sealability) and sealability of contraries, flatness, slipping character and the like.
Conventional packaging materials for photosensitive materials are a single layer film of high pressure branched low density polyethylene (LDPE) blended with carbon black, a composite laminated film composed of a LDPE resin film and a flexible sheet, such as paper or aluminum foil. An example of a conventional packaging material is, as illustrated in FIG. 8, composed of a LDPE resin film layer 12a containing a light-shielding material, a aluminum foil layer 11 and a bleached kraft paper layer 10 laminated in this order each through an adhesive layer 6.
As the packaging material, particularly a sealed lightproof bag, for a heavy roll or sheets of photographic photosensitive materials, the inventor has already disclosed a laminated film composed of a cross laminated film of uniaxially stretched high density polyethylene (HDPE) resin films excellent in physical strength and a LDPE resin film containing at least a light shielding material or an antistatic agent laminated thereto (Japanese Utility Model KOKOKU No. 19087/1981), and another packaging material also having a cross laminated film of which heat sealing properties and light-shielding ability are improved (Japanese Utility Model KOKOKU No. 20590/1986).
Another packaging material developed by the inventor is a laminated film having a film layer composed of linear low density polyethylene (L-LDPE) resin which is excellent in physical strength and heat sealing properties and is inexpensive that is blended with carbon black (U.S. Pat. No. 4,701,359 and Japanese Patent KOKAI No. 18547/1987).
As the packaging material having a metallized film, the inventor has already disclosed a packaging material for photosensitive materials composed of a metallized film layer and two L-LDPE resin polymer layers containing more than 50 wt. % of L-LDPE resin laminated on both sides thereof. At least, one of the L-LDPE resin polymer layers contains 0.3 to 30 wt. % of a light shielding material (U.S. Pat. No. 4,663,218).
The aforementioned conventional laminated film having a LDPE resin film layer is inferior in physical strength, such as tear strength, in spite of being thick, and in heat sealing properties. As a result, it was punctured, or the heat sealed portion was separated, during packaging or transportation. Since physical strength such as tear strength and tensile strength is remarkably improved by using the cross laminated film, the packaging materials containing the cross laminated film are suitable for heavy materials. Therefore, they were put to practical use up to recently. However, since the uniaxally stretched HDPE resin film layer is used as a heat seal layer, heat sealing properties are inferior. Moreover, physical strength varied due to unequal thickness of the adhesive layer and an unequal draw ratio. As a result, it was occasionally separated or punctured, and curling and twisting also occurred. The cross laminated film is expensive, because two kinds of film molding machines are necessary. The packaging materials having a L-LDPE resin film layer blended with carbon black are excellent in tear strength, impact puncture strength, heat sealing properties and the like, and they are suitable for packaging photosensitive materials. However, in the case of packaging heavy materials or materials having sharp edges, the packaging material were occasionally elongated due to a low Young's modulus, and thereby, light-shielding and moistureproofness were insufficient. The packaging material of U.S. Pat. No. 4,663,218 was improved in physical strength such as tear strength. However, when the L-LDPE resin film layer containing a light-shielding material was laminated on only one side of the metallized film layer, the packaging material was greatly curled. Moreover, when no antiblocking agent nor lubricant was blended into the layer, blocking occurred. Therefore, it has been put to practical use in a form that two L LDPE resin film layers were laminated on both sides of an aluminum vacuum metallized nylon film or polyester film having a large Young's modulus and heat resistance, though it was expensive.